This invention relates to impact-resistant polyamide molding compounds.
Polyamides are known and proven construction materials which can be processed, e.g., by injection molding or extrusion processes. In general, polyamides, especially after conditioning, exhibit good toughness. However, for certain uses, improvements with respect to impact strength and notch impact strength in the injection fresh condition (i.e., immediately after injection molding) and especially at low temperatures are desirable.
A suitable way of approaching this goal is the use of polyamide blends, i.e., intimate mixtures of polyamides with tough elastomers or tough, high-molecular weight thermoplastics. Special primary properties of these added polymers can thus be transferred to the polyamide blends without destroying the typical polyamide properties.
To obtain polyamide blends with satisfactory properties, in each case the polymers to be mixed in are optimized for the specific use and the respective polyamide type employed in the blend. Since such special polymers are not available on the market in the necessary variety, as a rule thee is the problem that in each case special products in mostly small amounts must be produced in a cost intensive way.
Because of these problems, in practice an alternative approach is taken in the production of polyamide-blend polymers which requires the use of olefinic polymers, available in large amounts and varieties, e.g., polyethylene or ethylene/propylene/diene copolymers, which exhibit a high cold or notch strength.
According to DE-PS No. 11 31 883, mixtures of (1) polycondensates containing carbonamide groups; (2) polyolefins, polystyrene, olefin or styrene copolymers, which, besides olefin or styrene, can also contain other olefinic unsaturated monomers incorporated by polymerization; and (3) catalyst forming radicals are homogenized at 50.degree. to 350.degree. C., whereby graft copolymers form from (1) and (2). However, the products thus obtained are not completely satisfactory, since cross-linking occurs, which results in inadequate processability.
The obviously preferred and most effective embodiment of the alternative approach discussed above consists of the grafting of high-molecular weight olefinic elastomers referably with, e.g., maleic anhydride, according to the working processes as described in DE-OS No. 24 01 149. However, such processes have the disadvantage that because of the action of maleic anhydride, which is greatly hazardous to health, the reaction cannot easily be performed in the usual industrial installations. Moreover, the effective elastomers are difficult to handle because of their tackiness, and because the aggregates possess a high shear strength and due to the thermal stress involved, the graft process often results in damages to the elastomers because of discolorations, decomposition and/or partial cross-linking.
All the products that can be used for the alternative method described above must exhibit a high molecular weight and consequently have high melting viscosities, if they are to produce good results in the polyamide blends. At the same time optimal properties and efficiency are attained only if the added polymers are finely dispersed in the polyamide matrix. This incorporation causes difficulties because of the high viscosity of the addition polymers and because of the unfavorable polyamide/added polymer viscosity relationship. Fluctuations in the flow rate and shearing conditions make the production of a reproducibly good quality difficult. Use of aggregates possessing a high shear strength does reduce the scattering of the values but requires a high energy expenditure and can result in damage to the polyamide blends.